Public Key Cryptography

The problems of key distribution are solved by public key cryptography, the
concept of which was introduced by Whitfield Diffie and Martin Hellman in
1975. (There is now evidence that the British Secret Service invented it a few
years before Diffie and Hellman, but kept it a military secret-and did nothing
with it.)

Public key cryptography is an asymmetric scheme that uses a pair of keys for
encryption: a public key, which encrypts data, and a corresponding private, or
secret key for decryption. You publish your public key to the world while
keeping your private key secret. Anyone with a copy of your public key can then
encrypt information that only you can read. Even people you have never met.

It is computationally infeasible to deduce the private key from the public key.
Anyone who has a public key can encrypt information but cannot decrypt it.
Only the person who has the corresponding private key can decrypt the
information.

The primary benefit of public key cryptography is that it allows people who
have no preexisting security arrangement to exchange messages securely. The
need for sender and receiver to share secret keys via some secure channel is
eliminated; all communications involve only public keys, and no private key
is ever transmitted or shared. Some examples of public-key cryptosystems are
Elgamal (named for its inventor, Taher Elgamal), RSA (named for its
inventors, Ron Rivest, Adi Shamir, and Leonard Adleman), Diffie-Hellman
(named, you guessed it, for its inventors), and DSA, the Digital Signature
Algorithm (invented by David Kravitz).

Because conventional cryptography was once the only available means for
relaying secret information, the expense of secure channels and key
distribution relegated its use only to those who could afford it, such as
governments and large banks (or small children with secret decoder rings).
Public key encryption is the technological revolution that provides strong
cryptography to the adult masses. Remember the courier with the locked
briefcase handcuffed to his wrist? Public-key encryption puts him out of
business (probably to his relief).